In Isaiah 58:1-9a, Isaiah explicitly says that God prefers a kind of fasting which changes our actual lifestyle and not just punishes our body. (The poor body is always the available scapegoat to avoid touching our purse, our calendar, or our prejudices.) Isaiah makes a very upfront demand for social justice, non-aggression, taking our feet off the necks of the oppressed, sharing our bread with the hungry, clothing the naked, sheltering the homeless, letting go of our sense of entitlement, and not speaking maliciously. He says very clearly this is the real fast God wants!
In Matthew 9:14-15, Jesus is asked why he and his disciples do not fast. In effect, he says “because it is the wrong kind of fasting!” Then he introduces a favorite theme and metaphor that he gradually develops: life as a wedding banquet, with himself as the bridegroom and humanity as the bride. It will soon become clear that Jesus is not interested in an elite who do their rituals properly yet refuse to join in the wedding feast that God is preparing for all, both insiders and outsiders.